1. Field of the Invention
The present invention relates generally to electronic component fabrication. More particularly, the present invention relates to methods and apparatus for placing a substrate, such as a semiconductor wafer or other bulk substrate, in contact with molten solder to fill apertures therein.
2. State of the Art
Semiconductor dice are conventionally produced with integrated circuits on a so-called “active surface” and may be configured to connect to other electronic components including other semiconductor dice as well as passive components forming, in combination, an electronic system. Conductively lined or filled apertures or holes (e.g., vias) and other electrical interconnects, such as bond pads, conductive traces and discrete conductive elements, such as solder or other conductive bumps, balls, studs, pillars and other structures, may be fabricated on a semiconductor die to facilitate connection to the other electronic components. One approach to effecting electrical connections includes the use of solder.
Wave soldering is a method used conventionally to form solder bumps on bond pads of semiconductor wafers, as well as connections on a printed circuit board (PCB), by placing a PCB in a substantially horizontal orientation (usually at a slight angle to the horizontal) in contact with a wave of molten solder that flows upwardly from a nozzle as the PCB is moved horizontally thereover. Horizontal wave soldering has been used to coat terminal pads and fill through holes extending between opposing surfaces of a PCB. Capillary action in combination with wettability of a metal lining of the through holes may be used to draw the molten solder into the through holes during the wave soldering process. Excess molten solder is removed from the PCB by moving the PCB up and away from the molten solder, so that gravity and surface tension separate the molten solder from that remaining on the PCB. In one conventional wave soldering process, the surfaces of the PCB that are to soldered are fluxed, the PCB is preheated, the solder wave is effectuated to contact the PCB, and an air knife is used to dislodge un-adhered solder from the surfaces of the PCB.
Although horizontal wave soldering is useful in coating terminal pads and filling through holes in PCBs, the process has exhibited certain limitations. For instance, the inclusion of surface mount components on the PCB and the continual decrease in size of line widths, spacing between adjacent through holes, and distance between leads makes efficient wave soldering difficult. Defects, such as solder skips resulting from a lack of sufficient solder or the formation of solder bridges and solder icicles resulting from excess solder, may exist and create quality issues during the manufacture of the PCBs.
Drag soldering involves lowering a pallet that supports a through-hole printed wiring board bearing electronic components having leads disposed in the through holes into a solder bath until the printed wiring board contacts the solder. The printed wiring board is dragged a predetermined distance along the surface of the solder bath, after which the printed wiring board is lifted from the bath. Excess solder drains from the printed wiring board after being removed from the solder bath. However, drag soldering often results in soldering defects, including bridging and the deposition of excess solder on the bottom side of the printed wiring board.
In hot air solder leveling (HASL), a PCB is fluxed and then contacted with heated, molten solder while in a vertical orientation such that fluxed conductors and connectors are solder coated. The PCB is brought into contact with the solder by immersion or dipping of the PCB into a solder pot or bath. After the PCB is removed from the molten solder, the PCB is passed between hot gas jets, knives, slots or orifices through which heated, compressed gas is flowing. The heated, compressed gas is at a pressure and temperature such that excess solder is removed and cleared from through-holes in the PCB while leaving solder on surface mount features thereon.
Other known processes of filling openings include chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless deposition, and electroplating. However, each of these processes may form voids or keyholes in the conductive filling of a via as the width of the via decreases in size. Further, these processes are time consuming and expensive.